Real-time radiation testing
Neutron radiation changes to materials can be tracked non-destructively. Ceri Jones spoke to the maker of a new system.
Radiation damage to materials can be continuously monitored using a new laser-based system developed by a research team at MIT, and Sandia National Laboratories, USA.
After devising the concept two years ago, the team has built and demonstrated a system able perform real-time and real-world testing, with the goal of designing materials that are more resistant to changes from radiation. This could save labour, resource and risk for material scientists in nuclear fields, where experiments involve radiological hazards.
Nuclear power stations across the USA sparked the idea. Commissioned around a similar period, they are now reaching their end-of-life, so must be assessed for extension or decommissioning. Materials analysis of the degree of neuron radiation exposure to the core structural components, elements that can’t be assessed without replacing the entire system, is long and expensive task, often involving destructive testing and microscopy. This new acoustic interrogation method provides non-contact, continuous monitoring, to pinpoint when material damage reaches critical levels.
‘As the materials see high radiation fields over the course of their operation lifetime, their properties will change drastically. That's a huge problem in radiation materials science, so the goal of our field is to design materials that are more resistant to these changes,’ MIT doctoral student, Cody Dennett, told Materials World.
‘We take a beam of charged heavy ions – in this case we're looking at metals so we try to use ions of the same metal – and we slam them into whatever we're looking at. We pulse the surface with a moderately high-intensity laser pulse to excite the surface but not damage it.
‘How fast that wave propagates, and other characteristics of how fast [the waves] decay away as a function of time can tell us things about the properties of the material we’re looking at. We monitor these things – the speed of how these waves propagate and how they decay.’
This interrogation provides an indication of a material’s elastic properties and how it transports heat. ‘We get this sort of two-mode picture of how evolution is occurring because different types of changes in the material will affect these two different properties in different ways, so it gives us two lenses to view what changes are happening through.
‘It may not give us exactly the same properties that we’d get back doing traditional destructive mechanical testing, but it gives us some indication of how the materials are changing, at what rate and where transition behaviour is happening in that evolution.’
Accelerated testing is not at the point where it can replace all traditional approaches, but it can offer advantages when used in conjunction. While ion beam exposure can take from hours up to a day to cause end-of-life equivalent damage seen in industrial environments, the time-consuming and essential data analysis can still take around a year. But the new system still has value, as Dennett explains.
‘Instead of having data points where, if you have a 10-year equivalent lifecycle and you’re sampling every year, taking 10 samples takes a year to analyse, we're sampling that whole 10-year equivalent lifecycle continuously as you’re applying the damage,’ he says. And while it doesn’t eradicate the need for destructive testing, ‘it does tell us where we want to do it – where do we actually carry out this time-consuming and resource-intensive interrogation to tell us the most about what’s happening?’.
Parallel applications are being proposed at present, as Dennett explains, building material knowledge will help inform future power plant designs and ‘new energy systems that don’t rely on these old technologies’. For those, you have starting points like stainless-steel and however you want to process it and change it, but really, to optimise the process for what is the absolute best choice we can make for these new systems has still not been made in a lot of cases. We can use [this] as a design tool for new systems that we’d like to put online, and also as a safety and reliability tool for systems that are in use and being life extended.’
Dennett and the team have been refining the methodology by testing samples of pure metals with no alloys or processing, like tungsten, and this week are testing samples of high entropy alloys as an intermediate material, before applying the learnings to engineering materials. While Dennett – builder and operator of the system – is already accepting samples from researchers to perform analysis, the piecemeal system will need to be rebuilt in a user-friendly model before the tool can be loaned out.
The paper, Real-time thermomechanical property monitoring during ion beam irradiation using in situ transient grating spectroscopy, was written by Dennett with Professor of Nuclear Science and Engineering Michael P Short, and Technologist Daniel L Buller of MIT, with Scientist Khalid Hattar of Sandia. It was published in Nuclear Instruments and Methods in Physics Research Section B, and can be read here: http://bit.ly/2U3Kq6R